- Email: [email protected]
Possibility of 8B solar neutrino detection via 19F inverse beta-decay reactions
4 September 1995
*H AsizJ ___
PHYSICS
LETTERS
A
El!!! ELSEVIER
Physics Letters A 205 (1995) 9-13
Possibility of ‘B solar neutrino detection via 19Finverse beta-decay reactions I.R. Barabanov, V.I. Cherehovsky, G.V. Domogatsky *, V.I. Gurentsov, A.B. Gurski, S.P. Mikheyev, I.V. Orekhov, V.G. Ryasny, G.T. Zatsepin Institute for Nuclear Research of the Russian Academy of Sciences, 60 October Anniversary Russian Federation
Prospekt,
7a, Moscow 117312,
Received 3 March 1995; revised manuscript received 5 July 1995; accepted for publication 5 July 1995 Communicated by J.P. Vigier
Abstract A method to detect ‘B solar neutrinos is proposed using the charge current reaction v,19F with a hexafluorobenzene scintillation detector. The main advantage of the detector is the possibility of identifying neutrino events by a delayed coincidence with the /3’ decay of 19Ne (Z’I,2 = 17 s). Because of this the detector is potentially sensitive to neutrinos with an energy as low as 5 MeV. An additional advantage of the detector is the ability to select unambiguously the electron neutrino component of the burst from a stellar gravitational collapse.
1. Introduction For solving the solar neutrino problem many hypotheses have been proposed, which can be divided into two groups: (i) the astrophysics of the sun proposed in the standard solar model is wrong or (ii) the standard model of electroweak interactions should be changed. No convincing results were available up to now and new experimental and theoretical investigations should be carried out. Among the second group of hypotheses the hypothesis of resonant neutrino oscillations, usually referred to as MSW effect [l], is most often discussed. In particular this effect should give rise to a change of the 8B neutrino spectrum which could be observed with an installa-
--i---E-mail: [email protected]. Elsevier Science B.V. SSDI 0375-9601(95)00509-9
tion sensitive to the neutrino spectrum with as low a detection threshold as possible. Investigations are being carried to develop a ‘B solar neutrino detector based on a hydrogenless fluorine scintillator (hexafluorobenzene, C, F6) thereafter HFB, with a specific density of 1.62 g/cm3) with a mass of 600 t (370 t of fluorine) [2]. The main task of the detector is to measure the 8B electron neutrino spectrum for neutrino energies above 5 MeV by detecting electrons produced in the 19F inverse betadecay reaction and the following 19Ne decay. The electron spectral shape analysis will enable one to distinguish the astrophysical solution of the solar neutrino problem (SNP) from the MSW effect in the range of small mixing angles including the case of electron neutrino conversion to the sterile state [3]. The second task is to detect the neutrino burst from stellar collapses. The delayed coincidence sig-
I.R. Barabanov et al./Physics Letters A 205 (1995) 9-13
10
nature of neutrino events enables one to select unambiguously the neutrino component of the burst, and to identify the initial stage of the burst. The fluorine detector also gives one the possibility to detect the total flux of neutrinos from the collapse independently of their flavour using the neutral current inelastic scattering reaction vX12 C + vX12 C *.
Table 1 Expected
2. Detector design
(“F is 100% natural fluorine). The threshold of the reaction is 3.24 MeV. We propose to detect electrons appearing in reaction (1) with energies above 2 MeV and also the lgNe decay CT,,* = 17.3 s),
An analogous principle of detector design was used for the Sudbury detector [4] and others. The scintillator is placed in the transparent vessel of a specially developed acrylic based on a usual polymethylmethacrylate coated by a thin layer (2 mm> chemically stable with regard to HFB acrylic. The acrylic vessel is suspended in a water shield inside a Kevlar net. The diameter of the vessel is 8.9 m and is intended for 600 t of HFB. The main requirements of the scintillator are: high scintillation output, high transparency for its own luminescence, short luminescence lifetime, possibility of (Y-P discrimination. At present the following composition of solutes is most attractive: 2 g/f of BP0 + 0.01-0.02 g/Z’ of d-POPOP. The light output for this composition is _ 16% of the antracene one for purified C,F,. The light transparency of the scintillator achieved now for the wavelength of 425 nm is 10 m, sufficient for creating the full-scale detector, but we hope to obtain a better transparency. We plan to use 1000 units of PMT “Quasar-370” with a photocathode diameter of 37 cm, which were developed for the Baikal project [5]. The PMTs will be made of low-background glass and placed in the water shield at a distance of 2 m from the vessel. The detector is considered to be placed in the underground room (4500 m.w.e., Baksan) and shielded from the radioactive background of the rock by a layer of low-background concrete (0.7 m) and water (3.5 m).
Threshold
rates for neutrino events (per year per 600 t) Reaction
SSM 5.8X106
37C1 2X106
K II 2.7X106
SSM MSW
ve 19 F
3490 490
1200 170
1630 230
1260 230
(MeV) 2 5.5
uxe-
“Ne + I9F+ e++ ve.
(2)
The average energy deposition of the “Ne decay is about 2 MeV including the positron annihilation energy. The energy range for the 19Ne decay detection will be 1.3-3.3 MeV (97% efficiency). Thus the neutrino events in the fluorine detector will be identified by three parameters: the energy deposition of the first and the second events, the time of appearance of the second event (5T,,, “Ne is supposed to be used) and the coincidence of the coordinates of both events within the detector spatial resolution. The second reaction type used for neutrino detection is v,e- scattering (v, refers to ve, v~, v,), v,+e--,
v,+e-.
(3) It is important that the fluorine detector enables us to measure the effect from reactions (1) and (3) independently due to the difference of signatures of the events. This enables us to compare correctly the flux of electron flavour neutrinos (reaction (1)) and the all flavour flux (reaction (3)). The expected rates of both reactions for SSM [6], the ‘B neutrino flux upper limit obtained in Davis’ experiment [7], the *B neutrino flux obtained from the results of Kamiokande II [81 and the MSW effect with the parameters rnf - m2 = 4 X 10e6 eV2 7 sin28 = 0.01 are shown 2 in Tablt 1.
3. Neutrino detection Neutrinos are detected via the reaction of the inverse p decay, “F+
v,-+“Ne+e-
(1)
* Current results are in accordance with our earlier analysis [Z] and slightly lower ( _ 6%) in comparision with Ref. 191. The difference is likely due to the Fermi function used.
f.R. Barabunou et al. /Physics
4. Search for *B neutrino oscillations Taking into consideration the results obtained up to now in the experiments on solar neutrino flux measurements [7,8,10,11] and considering the MSW effect in the range of small mixing angles with the conversion ve --f V~T or ve + vs (v, is the sterile neutrino state) as the SNP solution, the ‘B neutrino spectrum must have a shape different from the standard one in the case of any astrophysical solution 1121. Then the largest distortions have to be observed in the middle part of the *B neutrino spectrum within the energy range 4-8 MeV. The high energy resolution of the fluorine detector and the possibility of setting the detection threshold at the level of 2 MeV (N 5 MeV for neutrinos) enables us to measure the ‘B neutrino spectrum shape with sufficient accuracy for the true SNP solution to be selected. Spectra of the electrons from the *B neutrino in the fluorine detector taking into consideration the energy resolution and the detection efficiency which equals 0.85 are shown in Fig. 1 for two scenarios of the SNP solution: (i) MSW effect with the parameters rnf - rnf = 4 X 1O--6 eV2, sin28= 0.01; (ii> astrophysical solution normalized to the total number of events of spectrum 1 (corresponding flux 2.1 X 10” v/cm2 s).
Letters A 205 (19951 9-13
11
Table 2 Event rates (per year per 600 t) for the electron spectrum for the MSW effect (rni - mc = 4X 10e6 eV’ and sin*0 = 0.01) and spectrum of standard shape (astrophysical solution) normalized to the same number of events Energy range
MSW
Standard shape
2.0-6.2 6.2-12.0
456 610
533 533
To compare these spectra the event rates are given in Table 2 for two parts of the spectrum. An alternative method of searching for ‘B neutrino oscillations was realized in the Sudbury project [4] and is based on comparing the effects from the inverse P-decay reaction and the reaction via a neutral current. A similar method was considered in the BOREX project [13]. The measurement of the up and vr fluxes is also possible in the fluorine detector due to the possibility of detecting simultaneously and independently events from the inverse P-decay reaction and the ve,l.r,re- scattering reaction. It is essential that these measurements be carried out in the same detector and at the same period of time. In the case of the u + v oscillations both reactions will provide equaleBB neutrino fluxes while the electron spectrum detected via the inverse /3 decay reaction will have a nonstandard shape.
5. Neutrinos from a stellar gravitational collapse ---MSW
I
0
2
4
6 ENERGY,
8
10
12
Absence of hydrogen in the scintillator and the high cross-section of the neutrino reaction with 19F result in a selective sensitivity of the fluorine detector to electron neutrinos. Considering the possibility of identifying neutrino events via the 19Ne /3 decay the fluorine detector enables one to select unambiguously the electron neutrino component of the burst as the neutrino transparency stage lasts - 10 ms, and the deleptonization and cooling stages have duration of several tens of seconds. Besides reactions (1) and (3) the fluorine detector can detect the total neutrino flux with energy > 15.1 MeV via the reactions 114,151
MeV
Fig. 1. Electron energy spectra for the MSW effect with the ’ - rnz = 4 X 10m6 eV2, sin% = 0.01 and for the parameters 111, astrophysical solution of the SNP with the same total number of the events (sB neutrino flux of 2.1 X lo6 v/cm* s).
v+‘2c
-+Y*
12c
12
* --)
+ ZJ,
C + y (15.1 MeV),
where u= Us, V,, Z;L,$,
v,, 5,.
(4)
12
I.R. Barabanou et al. /Physics Number 01 pul8e~tMeV/600
0
5
10
tons
15
6. Background
20 At”DlitudO.
25 WV
30
35
40
The main background source for the v19F reaction are the spatial and time random coincidences of events from the internal radioactivity of the detector. The random coincidence background rate depends on the detector spatial resolution. The Monte Carlo calculations showed that 95% of the two-fold neutrino events could be localized in a sphere with a diameter of 1 m. The background rates from the internal radioactivity of the scintillator, acrylic vessel and the water shield were calculated for different concentrations of 238U, uzTh and 40K, 238U and 232Th being in equilibrium with their daughter products of decay. It was shown that the internal radioactivity background for reaction (1) is 6% of the neutrino effect at the ‘B neutrino flux of 2 X lo6 cmm2 s-’ and the radioactive element contamination for the scintillator is lo-l5 g/g U, Th and lo-l2 g/g K; for the vessel lo-l3 g/g U, Th and lo-” g/g K, and for the water shield lo-l4 g/g U, Th and 10-r’ g/g K. Another source of background for the fluorine detector is the radiation neutron capture reaction on 19F which could imitate the neutrino event [2],
45
Fig. 2. Amplitude spectra for detection of neutrino fluxes from a collapsing star in the fluorine detector: (1) inverse beta decay, (2) ve-scattering, (3) gamma-quanta from the neutral current carbon excitation.
The number of events in the detector is presented in Table 3 [16] for a collapsing star placed at a distance of 10 kpc and emitting 1O53 erg by v,, y, $7 VT:,V, and 1.1 X 1O53erg by ve. The 3, VP, v, and V, spectra were taken in Fermi-Dirac form with two variants of their parameters: (i) temperature kT = 8 MeV and chemical potential 7)= 0 spectrum 1 and (ii) kT = 6 MeV; 7 = 2.85kT spectrum 2 (see Ref. [16], and references therein). Pulse amplitude spectra for reactions (l), (31, (4) (spectrum 1) where calculated using the Monte Carlo code are shown in Fig. 2. Due to the high energy resolution of the scintillation detector and the possibility of identifying the v19F events the signal-noise ratio for the vX1’C reaction will be - 6. This is important for establishing limits of the VJ,~ masses by the time-of-flight method [17]. The hexafluorobenzene detector will be able to search for neutrino flavour oscillations by detecting charged current neutrino reactions with carbon [18].
Table 3 Number of neutrino interactions
Letters A 205 (1995) 9-13
from the gravitational
collapse
n+19F+20F*
(Cr N 6.6 MeV),
20F (Tl,2 = 11 s) -+20Ne* + e(E,,,
= 5.4 MeV) .
(5)
The main sources of neutrons in the scintillator are the l9F( (Y,n) reaction from internal natural cz-radioactivity and the spontaneous fission of =‘U. The background rate from reaction (5) was shown to be - 3 year-’ for the above concentrations of 238Uand 238Th, which is negligible in comparison with the neutrino effect. Cosmic rays also are a source of neutrons which after slowing down could generate reaction (5). Such
in the fluorine detector with a mass of 600 t
Reaction 19F + ve At9Ne
+ e-
vxe- + vxe-
V*12C+ Vx’2c *
20
28
Spectrum Total number of events
79
1
Spectrum 2 19
I.R. Barabanoo et al. /Physics
events may be identified by the passage of the muon through the scintillator and the water layer which will be the Cherenkov anticoincidence shield. The scintillator is supposed to include a boron liquid admixture (perhaps trimethylborate) at a level of 1% as an additional shield from neutrons.
Acknowledgement The research described in this publication was supported in part by Grant No. N6HOOO from the International Science Foundation.
References [l] S.P. Mikheyev and A.Yu. Smimov, Nuovo Cimento C 9 (1986) 17. [Z] I.R. Barabanov and G.V. Domogatsky, in: Proc. Sov. Conf. on Cosmic rays. Tashkent, Vol. 2, 1969, p. 77;
Letters A 205 (1995) 9-13
[3] [4] [5]
[6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18]
13
I.R. Barbanov, G.V. Domogatsky and G.T. Zatsepin, in: Proc. 13th Int. Conf. on Neutrino physics and astrophysics, Neutrino ‘88 (World Scientific, Singapore, 1989). V. Barger et al., Phys. Rev. D 43 (1991) 1759. G.T. Ewan et al., Sudbury Neutrino Observatory Proposal, SNO 87-12 (October 1987). RI. Bagduev et al., in: Proc. 2nd Int. Conf. on Trends in astroparticle-physics, Aachen, 1991, ed. P.Ch. Bosetti (Teubner, Leipzig, 1994) pp. 132-138. J.N. Bahcall and R. Ulrich, Rev. Mod. Phys. 60 (1988) 297. R. Davis et al., in: Proc. 21th ICRC, Adelaide, ed. R.J. Protheroe, Vol. 12, 1990, p. 143. K. Hirata et al., Phys. Rev. D 44 (1991) 2241. J.N. Bahcall, Neutrino astrophysics (19891 pp. 416-419. V.N. Gavrin et al., in: Proc. 26th ICHEP, Dallas, 1992, p. 1101. GALLEX collaboration, 8. Anselmann et al., Phys. Lett. B 285 (1992) 376. J.N. Bahcall, Phys. Rev. D 44 (1991) 1644. R.S. Raghavan et al., Phys. Rev. Lett. 57 (1986) 1801. W.C. Haxton, Phys. Rev. D 36 (1987) 2283. G. Badino et al., Nuovo Cimento C 7 (1984) 573. V.G. Ryasny, preprint INR RAS 803/93, 1993. G.T. Zatsepin, Pis’ma Zh. Eksp. Teor. Fiz. 8 (1968) 333. O.G. Ryazhskaya and V.G. Ryasny, Pis’ma Zh. Eksp. Teor. Fiz. 57 (1993) 195.
*H AsizJ ___
PHYSICS
LETTERS
A
El!!! ELSEVIER
Physics Letters A 205 (1995) 9-13
Possibility of ‘B solar neutrino detection via 19Finverse beta-decay reactions I.R. Barabanov, V.I. Cherehovsky, G.V. Domogatsky *, V.I. Gurentsov, A.B. Gurski, S.P. Mikheyev, I.V. Orekhov, V.G. Ryasny, G.T. Zatsepin Institute for Nuclear Research of the Russian Academy of Sciences, 60 October Anniversary Russian Federation
Prospekt,
7a, Moscow 117312,
Received 3 March 1995; revised manuscript received 5 July 1995; accepted for publication 5 July 1995 Communicated by J.P. Vigier
Abstract A method to detect ‘B solar neutrinos is proposed using the charge current reaction v,19F with a hexafluorobenzene scintillation detector. The main advantage of the detector is the possibility of identifying neutrino events by a delayed coincidence with the /3’ decay of 19Ne (Z’I,2 = 17 s). Because of this the detector is potentially sensitive to neutrinos with an energy as low as 5 MeV. An additional advantage of the detector is the ability to select unambiguously the electron neutrino component of the burst from a stellar gravitational collapse.
1. Introduction For solving the solar neutrino problem many hypotheses have been proposed, which can be divided into two groups: (i) the astrophysics of the sun proposed in the standard solar model is wrong or (ii) the standard model of electroweak interactions should be changed. No convincing results were available up to now and new experimental and theoretical investigations should be carried out. Among the second group of hypotheses the hypothesis of resonant neutrino oscillations, usually referred to as MSW effect [l], is most often discussed. In particular this effect should give rise to a change of the 8B neutrino spectrum which could be observed with an installa-
--i---E-mail: [email protected]. Elsevier Science B.V. SSDI 0375-9601(95)00509-9
tion sensitive to the neutrino spectrum with as low a detection threshold as possible. Investigations are being carried to develop a ‘B solar neutrino detector based on a hydrogenless fluorine scintillator (hexafluorobenzene, C, F6) thereafter HFB, with a specific density of 1.62 g/cm3) with a mass of 600 t (370 t of fluorine) [2]. The main task of the detector is to measure the 8B electron neutrino spectrum for neutrino energies above 5 MeV by detecting electrons produced in the 19F inverse betadecay reaction and the following 19Ne decay. The electron spectral shape analysis will enable one to distinguish the astrophysical solution of the solar neutrino problem (SNP) from the MSW effect in the range of small mixing angles including the case of electron neutrino conversion to the sterile state [3]. The second task is to detect the neutrino burst from stellar collapses. The delayed coincidence sig-
I.R. Barabanov et al./Physics Letters A 205 (1995) 9-13
10
nature of neutrino events enables one to select unambiguously the neutrino component of the burst, and to identify the initial stage of the burst. The fluorine detector also gives one the possibility to detect the total flux of neutrinos from the collapse independently of their flavour using the neutral current inelastic scattering reaction vX12 C + vX12 C *.
Table 1 Expected
2. Detector design
(“F is 100% natural fluorine). The threshold of the reaction is 3.24 MeV. We propose to detect electrons appearing in reaction (1) with energies above 2 MeV and also the lgNe decay CT,,* = 17.3 s),
An analogous principle of detector design was used for the Sudbury detector [4] and others. The scintillator is placed in the transparent vessel of a specially developed acrylic based on a usual polymethylmethacrylate coated by a thin layer (2 mm> chemically stable with regard to HFB acrylic. The acrylic vessel is suspended in a water shield inside a Kevlar net. The diameter of the vessel is 8.9 m and is intended for 600 t of HFB. The main requirements of the scintillator are: high scintillation output, high transparency for its own luminescence, short luminescence lifetime, possibility of (Y-P discrimination. At present the following composition of solutes is most attractive: 2 g/f of BP0 + 0.01-0.02 g/Z’ of d-POPOP. The light output for this composition is _ 16% of the antracene one for purified C,F,. The light transparency of the scintillator achieved now for the wavelength of 425 nm is 10 m, sufficient for creating the full-scale detector, but we hope to obtain a better transparency. We plan to use 1000 units of PMT “Quasar-370” with a photocathode diameter of 37 cm, which were developed for the Baikal project [5]. The PMTs will be made of low-background glass and placed in the water shield at a distance of 2 m from the vessel. The detector is considered to be placed in the underground room (4500 m.w.e., Baksan) and shielded from the radioactive background of the rock by a layer of low-background concrete (0.7 m) and water (3.5 m).
Threshold
rates for neutrino events (per year per 600 t) Reaction
SSM 5.8X106
37C1 2X106
K II 2.7X106
SSM MSW
ve 19 F
3490 490
1200 170
1630 230
1260 230
(MeV) 2 5.5
uxe-
“Ne + I9F+ e++ ve.
(2)
The average energy deposition of the “Ne decay is about 2 MeV including the positron annihilation energy. The energy range for the 19Ne decay detection will be 1.3-3.3 MeV (97% efficiency). Thus the neutrino events in the fluorine detector will be identified by three parameters: the energy deposition of the first and the second events, the time of appearance of the second event (5T,,, “Ne is supposed to be used) and the coincidence of the coordinates of both events within the detector spatial resolution. The second reaction type used for neutrino detection is v,e- scattering (v, refers to ve, v~, v,), v,+e--,
v,+e-.
(3) It is important that the fluorine detector enables us to measure the effect from reactions (1) and (3) independently due to the difference of signatures of the events. This enables us to compare correctly the flux of electron flavour neutrinos (reaction (1)) and the all flavour flux (reaction (3)). The expected rates of both reactions for SSM [6], the ‘B neutrino flux upper limit obtained in Davis’ experiment [7], the *B neutrino flux obtained from the results of Kamiokande II [81 and the MSW effect with the parameters rnf - m2 = 4 X 10e6 eV2 7 sin28 = 0.01 are shown 2 in Tablt 1.
3. Neutrino detection Neutrinos are detected via the reaction of the inverse p decay, “F+
v,-+“Ne+e-
(1)
* Current results are in accordance with our earlier analysis [Z] and slightly lower ( _ 6%) in comparision with Ref. 191. The difference is likely due to the Fermi function used.
f.R. Barabunou et al. /Physics
4. Search for *B neutrino oscillations Taking into consideration the results obtained up to now in the experiments on solar neutrino flux measurements [7,8,10,11] and considering the MSW effect in the range of small mixing angles with the conversion ve --f V~T or ve + vs (v, is the sterile neutrino state) as the SNP solution, the ‘B neutrino spectrum must have a shape different from the standard one in the case of any astrophysical solution 1121. Then the largest distortions have to be observed in the middle part of the *B neutrino spectrum within the energy range 4-8 MeV. The high energy resolution of the fluorine detector and the possibility of setting the detection threshold at the level of 2 MeV (N 5 MeV for neutrinos) enables us to measure the ‘B neutrino spectrum shape with sufficient accuracy for the true SNP solution to be selected. Spectra of the electrons from the *B neutrino in the fluorine detector taking into consideration the energy resolution and the detection efficiency which equals 0.85 are shown in Fig. 1 for two scenarios of the SNP solution: (i) MSW effect with the parameters rnf - rnf = 4 X 1O--6 eV2, sin28= 0.01; (ii> astrophysical solution normalized to the total number of events of spectrum 1 (corresponding flux 2.1 X 10” v/cm2 s).
Letters A 205 (19951 9-13
11
Table 2 Event rates (per year per 600 t) for the electron spectrum for the MSW effect (rni - mc = 4X 10e6 eV’ and sin*0 = 0.01) and spectrum of standard shape (astrophysical solution) normalized to the same number of events Energy range
MSW
Standard shape
2.0-6.2 6.2-12.0
456 610
533 533
To compare these spectra the event rates are given in Table 2 for two parts of the spectrum. An alternative method of searching for ‘B neutrino oscillations was realized in the Sudbury project [4] and is based on comparing the effects from the inverse P-decay reaction and the reaction via a neutral current. A similar method was considered in the BOREX project [13]. The measurement of the up and vr fluxes is also possible in the fluorine detector due to the possibility of detecting simultaneously and independently events from the inverse P-decay reaction and the ve,l.r,re- scattering reaction. It is essential that these measurements be carried out in the same detector and at the same period of time. In the case of the u + v oscillations both reactions will provide equaleBB neutrino fluxes while the electron spectrum detected via the inverse /3 decay reaction will have a nonstandard shape.
5. Neutrinos from a stellar gravitational collapse ---MSW
I
0
2
4
6 ENERGY,
8
10
12
Absence of hydrogen in the scintillator and the high cross-section of the neutrino reaction with 19F result in a selective sensitivity of the fluorine detector to electron neutrinos. Considering the possibility of identifying neutrino events via the 19Ne /3 decay the fluorine detector enables one to select unambiguously the electron neutrino component of the burst as the neutrino transparency stage lasts - 10 ms, and the deleptonization and cooling stages have duration of several tens of seconds. Besides reactions (1) and (3) the fluorine detector can detect the total neutrino flux with energy > 15.1 MeV via the reactions 114,151
MeV
Fig. 1. Electron energy spectra for the MSW effect with the ’ - rnz = 4 X 10m6 eV2, sin% = 0.01 and for the parameters 111, astrophysical solution of the SNP with the same total number of the events (sB neutrino flux of 2.1 X lo6 v/cm* s).
v+‘2c
-+Y*
12c
12
* --)
+ ZJ,
C + y (15.1 MeV),
where u= Us, V,, Z;L,$,
v,, 5,.
(4)
12
I.R. Barabanou et al. /Physics Number 01 pul8e~tMeV/600
0
5
10
tons
15
6. Background
20 At”DlitudO.
25 WV
30
35
40
The main background source for the v19F reaction are the spatial and time random coincidences of events from the internal radioactivity of the detector. The random coincidence background rate depends on the detector spatial resolution. The Monte Carlo calculations showed that 95% of the two-fold neutrino events could be localized in a sphere with a diameter of 1 m. The background rates from the internal radioactivity of the scintillator, acrylic vessel and the water shield were calculated for different concentrations of 238U, uzTh and 40K, 238U and 232Th being in equilibrium with their daughter products of decay. It was shown that the internal radioactivity background for reaction (1) is 6% of the neutrino effect at the ‘B neutrino flux of 2 X lo6 cmm2 s-’ and the radioactive element contamination for the scintillator is lo-l5 g/g U, Th and lo-l2 g/g K; for the vessel lo-l3 g/g U, Th and lo-” g/g K, and for the water shield lo-l4 g/g U, Th and 10-r’ g/g K. Another source of background for the fluorine detector is the radiation neutron capture reaction on 19F which could imitate the neutrino event [2],
45
Fig. 2. Amplitude spectra for detection of neutrino fluxes from a collapsing star in the fluorine detector: (1) inverse beta decay, (2) ve-scattering, (3) gamma-quanta from the neutral current carbon excitation.
The number of events in the detector is presented in Table 3 [16] for a collapsing star placed at a distance of 10 kpc and emitting 1O53 erg by v,, y, $7 VT:,V, and 1.1 X 1O53erg by ve. The 3, VP, v, and V, spectra were taken in Fermi-Dirac form with two variants of their parameters: (i) temperature kT = 8 MeV and chemical potential 7)= 0 spectrum 1 and (ii) kT = 6 MeV; 7 = 2.85kT spectrum 2 (see Ref. [16], and references therein). Pulse amplitude spectra for reactions (l), (31, (4) (spectrum 1) where calculated using the Monte Carlo code are shown in Fig. 2. Due to the high energy resolution of the scintillation detector and the possibility of identifying the v19F events the signal-noise ratio for the vX1’C reaction will be - 6. This is important for establishing limits of the VJ,~ masses by the time-of-flight method [17]. The hexafluorobenzene detector will be able to search for neutrino flavour oscillations by detecting charged current neutrino reactions with carbon [18].
Table 3 Number of neutrino interactions
Letters A 205 (1995) 9-13
from the gravitational
collapse
n+19F+20F*
(Cr N 6.6 MeV),
20F (Tl,2 = 11 s) -+20Ne* + e(E,,,
= 5.4 MeV) .
(5)
The main sources of neutrons in the scintillator are the l9F( (Y,n) reaction from internal natural cz-radioactivity and the spontaneous fission of =‘U. The background rate from reaction (5) was shown to be - 3 year-’ for the above concentrations of 238Uand 238Th, which is negligible in comparison with the neutrino effect. Cosmic rays also are a source of neutrons which after slowing down could generate reaction (5). Such
in the fluorine detector with a mass of 600 t
Reaction 19F + ve At9Ne
+ e-
vxe- + vxe-
V*12C+ Vx’2c *
20
28
Spectrum Total number of events
79
1
Spectrum 2 19
I.R. Barabanoo et al. /Physics
events may be identified by the passage of the muon through the scintillator and the water layer which will be the Cherenkov anticoincidence shield. The scintillator is supposed to include a boron liquid admixture (perhaps trimethylborate) at a level of 1% as an additional shield from neutrons.
Acknowledgement The research described in this publication was supported in part by Grant No. N6HOOO from the International Science Foundation.
References [l] S.P. Mikheyev and A.Yu. Smimov, Nuovo Cimento C 9 (1986) 17. [Z] I.R. Barabanov and G.V. Domogatsky, in: Proc. Sov. Conf. on Cosmic rays. Tashkent, Vol. 2, 1969, p. 77;
Letters A 205 (1995) 9-13
[3] [4] [5]
[6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18]
13
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